The present invention relates to a planar blank intended for the manufacture of a coil spring. The coil spring being suitable for transluminal implantation. The invention also includes the coil spring being manufactured from the blank.
In surgical and other medicinal techniques there is often a need for being able to insert and expand devices in for example blood vessels, urinary tracts, respiratory tracts, intestinals or other difficultly accessible locations. The devices having for their function to widen the said vessel or duct and being capable of being left in position for providing permanent stability at the site of implantation.
In published British patent application No. 83 26791 there is disclosed a device which has for its purpose to provide the function indicated above. In this known device used for mechanical transluminal implantation, the prosthesis or spring means provides self-fixation by self-expansion at the site of implantation. There are, however, certain practical problems in connection with the transluminal displacement. When inserted into for example a blood vessel, the known coil spring is wound onto an elongated rod-shaped element and is there after released at the desired location in the body. In this operation it is essential that the coil spring surround the carrier body very closely so that the surrounding walls of the vessel will not be damaged during the transluminal movement of the device. It is also essential that the coil spring when released and expanded at the site of implantation will be anchored in a reliable manner so that it will not later move from its position at the site of implantation. The known device does not operate wholly satisfactorily in these two respects, and the present invention thus has for its purpose to provide an improved coil spring for transluminal implantation and a planar blank from which the coil spring is manufactured.
For this purpose the planar blank according to the present invention comprises an elongated, essentially straight midsection which, at opposite endsections is bent in a plane of the blank in opposite directions to form a blank having an approximate Z-shape. In this manner, a coil spring is made from the blank. At its end sections, the coil spring has a lower pitch than at the midsection of the coil spring and, in fact, the coil spring may have no pitch or substantially no pitch at all at its ends. By use of the planar bank, the material of the manufactured spring has a radially flattened shape, as seen in crossection.
For the purpose of enabling lower pitch and smaller distance between the turns in the midsection of the coil spring, the midsection of the blank can be made smaller than the endsections when viewed in the plane of the blank.
In order that the coil spring according to the invention, shall find use as a filter for thromboses, for example by application in Vena Cava Inferior to prevent formation of lungemboli, one endsection of the blank according to the invention can be extended as compared to the other endsection, so that when a coil spring is made from the blank several overlapping turns are formed. The turns, when viewed in a direction perpendicular to the centre line of the spring, lie essentially in the same plane. In this context it is preferred that one endsection be provided as an extended endsection and is made outwardly tapering for the purpose of reducing the flow resistance of the coil spring made from the blank.
In an alternative embodiment of the blank according to the invention, both endsections are substantially of equal length and have a length less than the length of a complete turn in the coil spring made from the blank when in an unloaded state. The endsection length is suitably less than about 3/4 and preferably about 1/2 of said complete turn length.
The blank according to the invention is, preferably, over at least the major part of its length, provided with apertures extending in a direction perpendicular to the plane of the blank. The apertures are preferably arranged in at least one row extending in the longitudinal direction of the blank.
For providing a suitable shape of the coil spring made from the blank, the endsections of the blank are suitably bent over an angle (.alpha.) lying within the range about 5.degree.-40.degree., preferably about 10.degree.-30.degree. and particularly about 10.degree.-25.degree..
The coil spring according to the present invention has an essentially helix shaped configuration, and its spring material as seen in an axial crossection is radially flattened. After being manufactured from the blank, the coil spring is characterized by a midsection having a circular cylindric shape and endsections having, in relation to the midsection, lower pitch or zero pitch. In a corresponding manner as the blank according to the invention said midsection of the coil spring may be made from material that is of an axially smaller dimension than the material of the endsections.
For use as a filter, the coil spring may have one endsection consisting of several turns which, in a direction perpendicular to the centre line of the coil spring lie in essentially the same plane, and which, in a direction towards the free end of the endsection have a progressively decreasing radius of curvature. In order to reduce the axial flow resistance of such coil spring the material of said one endsection is suitably made axially tapering in a direction extending towards the free end.
The coil spring according to the present invention may also be made substantially symmetrical, the material of the two endsections having essentially the same length which is less than a full turn of the spring and preferably less than 3/4 turn of the spring.
For the purpose of facilitating the ingrowth of the coil spring in for example a blood vessel and thus to facilitate tissue growth the spring material over at least the major part of the spring is suitably provided with radially through-going apertures. It is particularly preferred that said openings have an area such that the main part of the remaining spring material has a tangential width of at most about 2.5 mm, preferably at most about 2 mm and particularly at most about 1.5 mm.
The material of the blank and the coil spring according to the present invention can be any medicinally acceptable material but is preferably constituted by a medicinally acceptable metal, for example of the type cobalt, chromium, nickel, molybdenum, iron alloy as defined in International Standard, ISO 5832/7-1984 (E). One example of such medicinally preferred metal alloy is Phynox.RTM. which alloy contains nickel, chromium, molybdenum, cobalt, silicon and manganese in an amount of 18, 20, 7, 40, 0.3 and 1.5% by weight respectively.
The coil spring and the blank according to the present invention offer substantial advantages as compared to the prior art. Thus, the coil spring manufactured from the blank according to the invention may be easier manufactured by winding on to a mandrel of a suitable diameter as the bent endsections enable flat and even winding on to the mandrel. Moreover, the coil spring in transluminal implantation offers the major advantage that it can be wound on to a tubular or rod-shaped body to tight engagement thereon so that the transluminar displacement can be carried out without damages to surrounding tissue.
With regard to the thickness of the planar blank according to the invention it is, of course, dependent on the size of the coil spring to be manufactured and may therefor vary within broad limits. However, the preferred thickness range is about 0.05 to 0.7 mm, preferably up to 0.5 mm. A particularly useful thickness range is about 0.1 to about 0.3 mm.